In many cases, a drug prescribed for an inpatient in a hospital is obtained by taking out several kinds of drugs from different drug containers to mix the drugs. In addition, such a drug mixing operation is often manually performed by an operator, such as a pharmacist. To mix the drugs, an operation of inserting an injection needle into each of the drug containers to suck the drug therein by means of a syringe is necessary. This operation is a work load on the operator.
For instance, when a sealed vial container is used as the drug container to suck the drug from the drug container into the syringe, a pumping operation is often performed. The pumping operation is an operation in which suction is started in a state where an air layer having a smaller volume than the drug to be sucked is previously reserved in the syringe, thereby pushing and pulling the plunger of the syringe over and over again. When the pumping operation is performed, the drug in the drug container is gradually substituted for an air. The pumping operation is an operation which takes time and is troublesome for the operator, such as a pharmacist. In addition, when a viscosity of the drug in the drug container is high, viscosity resistance when the drug passes through the injection needle is increased. Therefore, the operator needs a large force.
In this way, the operation of sucking the drug from the drug container into the syringe needs time and labor, resulting in a great work load on the operator. Likewise, when the drug is discharged from the syringe into the drug container, first, a gas in the drug container is slightly sucked, thereby gradually substituting a liquid in the syringe for the gas in the drug container. The operation has also a great work load on the operator.
To reduce this work load, a drug transferring needle in which an air vent slot is provided in part of the needle to make the pumping operation unnecessary is proposed (for instance, see Patent Literature 1). In the conventional drug transferring needle, the air vent slot is provided in part of the needle, so that a pressure in the drug container and an atmospheric pressure are substantially equal.
FIG. 11 is a partially sectional view of a conventional drug transferring needle 1. In the conventional drug transferring needle 1, a metal needle 2 and a needle base 3 are integral. In the drug transferring needle 1, a sheath 4 is partially fitted onto an outer side of the metal needle 2, and an air vent slot 4a is formed between the metal needle 2 and the sheath 4. In addition, in the drug transferring needle 1, a filter 6 formed of a hydrophobic synthetic resin is attached to near an air vent opening 4b at one end of the air vent slot 4a. 
The drug transferring needle 1 is attached to a syringe 7. Therefore, the pressure in the drug container and the atmospheric pressure are substantially equal, thereby making the pumping operation unnecessary. The reason why the pumping operation is unnecessary will be described.
When the drug transferring needle 1 is needled into a rubber stopper of a drug container 8 to suck a drug 9 from the drug container 8 into the drug transferring needle 1, part of the drug 9 in the drug container 8 is sucked into the syringe 7 through the drug transferring needle 1. As a result, the drug 9 in the drug container 8 is reduced. At this time, a gas 10 in the drug container 8 expands by a reduced volume of the drug 9. Therefore, a pressure of the gas 10 in the drug container 8 is lowered. Then, a pressure difference occurs between the pressure of the gas 10 in the drug container 8 and the atmospheric pressure. Outside air is then drawn into the drug container 8 through the filter 6 and the air vent slot 4a of the drug transferring needle 1 so as to reduce the pressure difference. In this way, a bubbled air 5a enters into the drug container 8 through the air vent slot 4a of the drug transferring needle 1. Therefore, the pressure difference is eliminated so that the pressure of the gas 10 in the drug container 8 is substantially equal to the atmospheric pressure. In this way, when the drug transferring needle 1 is used, the pressure of the gas 10 in the drug container 8 is regulated to be substantially equal to the atmospheric pressure. With this, the pumping operation of the syringe 7 can be unnecessary, and the work load when the drug is sucked from within the drug container 8 can be reduced.